1. Field of the Invention
The present invention relates to a sedimentation centrifuge for separating a mixture of solid particles and liquids into solid and liquid phases, and more particularly to a sedimentation centrifuge for separating a mixture of solid particles and two liquid phases, i.e., heavy and light liquids, into the solid particles, the heavy liquid, and the light liquid.
2. Description of the Prior Art
Heretofore, centrifuges or centrifugal separators are used to separate a mixture of solid particles and liquids into solid and liquid phases, or a mixture of solid particles and heavy and light liquids into the solid particles, the heavy liquid, and the light liquid.
Disk decanters (also called cone-disk decanters) which are in general use for centrifugal sedimentation have a number of spaced conical disks in the form of truncated cones. When a feed slurry of solid-liquid mixture with a large solid content is separated by a disk decanter, however, accumulated solid particles in the mixture tend to clog the gaps between the disks, making the disc decanter fail to operate. To prevent such a problem from happening, it is necessary to reduce the amount of solid particles contained in the mixture with, for example, a screw decanter, and then separate the mixture into solid and liquid phases with a disk decanter. However, this practice requires two expensive decanters connected in series with each other. Furthermore, since the mixture has to be processed under intensive centrifugal force twice, the energy consumed by the decanters is relatively large and wasteful
The inventor has proposed a sedimentation centrifuge for separating a mixture of solid particles and liquids into solid and liquid phases, and also a sedimentation centrifuge for separating a mixture of solid particles and two liquid phases, i.e., heavy and light liquids, into the solid particles, the heavy liquid, and the light liquid, as disclosed in Japanese Patent No. 1,007,732 (Japanese Patent laid-Open No. 054961/75) and Japanese Patent Laid-Open No. 152556/87. The proposed sedimentation centrifuges, which may be termed doubly-canted screw decanters, have a frustoconical bowl for discharging sedimented solid particles. The bowl has an inner wall surface that is canted with respect to the central axis of the bowl first at a larger angle and then at a smaller angle along the direction in which the accumulated solids are discharged. The doubly-canted inner wall surface of the bowl allows the solids to travel smoothly to the end of the bowl and also allows liquid to be removed smoothly from the solid particles. The bowl structure also serves to increase the amount of feed that stays in the centrifuge for an increased period of time. Therefore, the separation accuracy of the centrifuge and the processing capacity of the centrifuge per unit time are increased.
The inventor has found that much remains to be improved in terms of the processing capacity, the separation accuracy, and the amount of required energy in the practical application of the centrifuges. More specifically, as described above, the disk decanter often fails to operate because sedimented solids tend to clog the gaps between the disks particularly when the feed has a large solid content. This trouble can only be avoided by first reducing the solid content in the slurry with the screw decanter and then processing the slurry with the disk decanter. Therefore, the two expensive decanters or centrifuges need to be connected in series with each other, resulting in a wasteful consumption of energy as intensive centrifugal forces are required to be applied twice in the decanters. While the doubly-canted screw decanters proposed by the inventor can finish the separating process when the slurry passes through the centrifuge only once, the actual use of the proposed decanters has indicated that the proposed decanters are not so advantageous as expected over the series-connected centrifugal separators and need further improvement with respect to the processing capacity and the energy consumption in applications which demand accurate phase separation.